Generally, in a compact disc (CD) and a digital versatile disc (DVD), binary signals are recorded and tracking signals are detected by using a change in a reflectance that is generated by interference between lights from a bottom of a recessed pit and a mirror surface portion. In recent years, a phase-change type rewritable compact disc (CD-RW: CD-Rewritable) has become widely used. Additionally, various kinds of phase-change type rewritable DVD have also been suggested. Further, while a capacity of a DVD is 4.7 GB, a system high-density BD has been suggested, which has a capacity of more than 20 GB by reducing a recording and reproduction wavelength to 350 nm to 420 nm and increasing a numerical aperture NA (for example, refer to Non-Patent Document 1). Additionally, in a DVD-R, dual-layer media have been suggested and a recording or reproduction apparatus compliant with the dual-layer media has been achieved.
In the phase-change type rewritable CD, DVD and BD, recording information signals are detected by using a reflectance change and a phase-difference change generated by a difference in refractive index between an amorphous state and a crystal state. A normal phase-change type recording medium has a structure in which a lower protective layer, a phase-change type recording layer, an upper protective layer and a reflection layer are provided on a substrate. The reflectance difference and the refractive index difference are controlled by utilizing multiple interaction of those layers so as to give compatibility with a CD or a DVD. In a CD-RW, a compatibility of recording signals and groove signals with a CD can be acquired within a range of a reflectance reduced to 15% to 25%. Accordingly, reproduction can be made by using a CD drive having an amplification system that covers a low reflectance. It should be noted that in the phase-change type optical recording medium, erasure and rerecording can be performed by intensity modulation of a single convergent light beam. Thus, in a phase-change type optical recording medium such as a CD-RW or a rewritable DVD, recording may include overwriting (O/W) that simultaneously performs recording and erasure in many cases.
As for recording information using a phase change, a crystal, an amorphous or a mixture of those can be used, and also a plurality of crystal phases can be used. In the phase-change type optical recording media materialized at present, it is general that an unrecorded or erasure state is made into a crystal state and recording is made by forming marks of an amorphous state.
As a material of a phase-change type recording layer, a chalcogen element, i.e., a chalcogenide based alloy containing S, Se and Te is used in many cases. For example, they are a GeSbTe system containing a GeTe—Sb2Te3 pseudo binary alloy as a major component, an InSbTe system containing an InTe—Sb2Te3 pseudo binary alloy as a major component, an AgInSbTe system containing a Sb0.7Te0.3 eutectic alloy as a major component, a GeSbTe system, etc. Among those, a system in which superfluous Sb is added to the GeTe—Sb2Te3 pseudo binary alloy, especially an intermetallic compound neighborhood composition such as Ge1Sb2Te4 or Ge2Sb2Te5 is mainly put into practice. Those compositions features crystallization without phase separation that is peculiar to an intermetallic compound, and is easily initialized since a crystal growth rate is large and a recrystallization rate is large during erasure. Thus, conventionally, as a recording layer exhibiting a practical O/W characteristic, a pseudo binary alloy system and an intermetallic compound neighborhood composition attracted attention (for example, refer to Non-Patent Document 2).
Moreover, conventionally, reports are made regarding a recording layer composition, which is a GeSbTe ternary composition or the ternary composition as a matrix containing an additive element (for example, refer to Patent Documents 1-4).
However, a material and a phase composition dealing with the dual-layer media of a DVD-RW is in future development, and there are many problems in applying such a material and phase composition to an optical recording medium for high-density recording such as a rewritable BD.
There is suggested as a heat resistant protective layer material a metal oxide such as ZnO or ZrO, a carbide such as TiC or a mixture of the aforementioned. However, there is no effect of satisfying a recording and reproduction characteristic in sulfide resistance while acquiring storage stability (for example, refer to Patent Document 5).
Moreover, there is suggested a single recording layer with a layer having a triple layer composition of ZnS.SiO2/AlN/ZnS.SiO2 provided between the recording layer and a substrate (for example, refer to Non-Patent Document 3).
However, a total thickness of the layer cannot be reduced after all.
Moreover, the applicant suggested a phase-change type optical recording medium having a single recording layer structure in which a first recording composition layer, a resin intermediate layer and a second recording layer provided on a substrate in that order (refer to Patent Document 6). The first recording composition layer consists of a heat release layer, a first interface layer, a first protective layer, a first recording layer and a second protective layer in that order from a substrate side. The second recording composition layer consists of a heat release layer, a second interface layer, a fourth protective layer, a second recording layer and a fifth protective layer in that order from the resin intermediate layer side.
Moreover, there is suggested an optical information medium having a first recording stack layer and a second recording stack layer provided in that order on one side of a substrate (for example, refer to Patent Document 7). In this optical information medium, the first recording stack layer consists of a recording layer sandwiched between two dielectric material layers, a transparent metal layer, and a further dielectric layer on a side opposite to a side on which a laser light beam is incident. The second recording stack layer consists of a transparent spacer layer, a phase-change type recording sandwiched between two dielectric material layers, and a metal mirror layer on a side opposite to a side on which the laser light beam is incident. In this case, the first recording stack layer has a thickness of 10 nm to 30 nm, and the metal layer of the first recording stack layer is made of silver. Moreover, the two recording layers are made of recordable material, and both layers are sandwiched by protective layers. Further, the thickness of the metal layer is less than 10 nm and equal to or more than 2 nm.
Patent Document 1: Japanese Laid-Open Patent Application No. 61-258787
Patent Document 2: Japanese Laid-Open Patent Application No. 62-152786
Patent Document 3: Japanese Laid-Open Patent Application No. 64-63195
Patent Document 4: Japanese Laid-Open Patent Application No. 1-211249
Patent Document 5: Japanese Laid-Open Patent Application No. 6-60426
Patent Document 6: Japanese Laid-Open Patent Application No. 2004-185794
Patent Document 7: Japanese Laid-Open Patent Application No. 2002-515623
Non-Patent Document 1: ISOM Technical Digest, '00 (2000), pp. 210
Non-Patent Document 2: SPIE, vol. 2514 (1995), pp. 294-301
Non-Patent Document 3: ODS2001, Technical Digest pp. 28
However, although the above-mentioned phase-change type optical recording media are easily recordable with a short wavelength such as a blue laser since the material absorbs a large amount of light of a short wavelength, it is difficult to record the media with a red wavelength since the material absorbs only a small amount of light of the red wavelength. Thus, in a medium having a single recording layer, a thick reflection layer is provided so as to prevent a light from transmitting therethrough so that the media is easily recordable.
However, in a case where two recording layers are provided, multi-layer recording cannot be performed unless the first recording layer on the side where a light is incident transmits about a half amount of the light. Thus, if the first recording layer is made to transmit the light therethrough, the first layer cannot absorb a sufficient amount of light and it is difficult to achieve recording. Since a red wavelength LD is capable of emitting a light with a high power, one time recording can be performed by applying a high power to the medium. However, it is difficult to maintain an overwrite (O/W) characteristic since the film tends to be deteriorated by the high power being applied. Additionally, if a recording sensitivity is raised so as to be easily overwritten, there is a problem that reliability with respect to temperature and humidity is deteriorated.
In the meantime, DVDs currently on the market are classified into a DVD (containing a dual-layer structure) that is exclusive for reproduction, a DVD±R (containing a dual-layer structure) that is recordable, and a DVD±RW (only a single layer structure) containing dual-layer structure) that is rewritable. Any of those DVDs can be reproduced by a DVD video recorder or a DVD player. However, presently, the rewritable DVD±RW is materialized with only a single layer structure, and recording time is about a half of that of a dual-layer structure, which raises a problem in that long time recording cannot be made. Thus, it is desirous to develop a DVD±RW, which is rewritable and permits long time recording.
However, a reflectance of a dual-layer structure DVD±RW is only about one-third of that of a recordable dual-layer structure DVD±R. Accordingly, the dual-layer structure DVD±RW cannot be reproduced by a present DVD video recorder or a DVD player. Thus, it is necessary to devise in an information reproducing method including a composition of a DVD.
In an optical disc apparatus technique, a focus error signal (FE) and a track error signal (TE) are normalized so as to perform a focus servo and a track servo, and a technique for normalization is suggested (for example, refer to Patent Document 8). Specifically, Patent Document 8 discloses that a summation signal from a divided light-receiving element (hereinafter, referred to as PU) of an optical pickup device is used as an original signal for normalizing a servo signal such as a focus signal, a track signal, etc.
FIG. 1 shows an automatic gain control (AGC) circuit used in a conventional optical disc apparatus. In the AGC circuit, a plurality of signals VA, VB, VC and VD from a PU are operated by a focus error (FE) arithmetic circuit 101 to calculate FE=(A+C)−(B+D) and a track error (TE) arithmetic circuit 102 to calculate TE=(B+C)−(A+D). Additionally, SUM=(A+B+C+D) is calculated by a SUM arithmetic circuit 103. A normalization circuit comprises an AGCCNT circuit 104 and voltage control amplifiers (VCAs) 105. A gain of each VCA 105 is set by the AGCCNT circuit 104 so that a level of a SUM signal from the SUM arithmetic circuit 103 input to the AGCCNT circuit 104 is a constant voltage. Specifically, for example, if it is set so that the level of the SUM signal is 1 V, the gain of each VCA 105 becomes equal to 1 (GAIN=1/SUM).
At this time, an AGC operation of the FE signal and the TE signal is carried out as follows.FEn=[(A+C)−(B+D)]/(A+B+C+D)TEn=[(B+C)−(A+D)]/(A+B+C+D)
It is quite difficult to enlarge an AGC gain range in each VCA 105, and about ±10 dB is appropriate. Since the AGC amplifier is used for a servo signal, an input offset cannot be fluctuated with respect to a gain fluctuation. That is, an extremely small fluctuation is required for a large gain range.
Patent Document 8: Japanese Laid-Open Patent Application No. 2001-101680
Therefore, in the conventional optical disc apparatus shown in FIG. 1, in order to take a large range of the gain and to also reduce a fluctuation of an offset, a circuit scale must be enlarged and there is a problem that realization is difficult. Moreover, although an input offset and an AGC gain error are small at the center of the AGC range, the input offset and the AGC gain error become large at the end of the AGC range.